It has been found useful to employ optical delivery devices that include optical fibers and deliver an output beam of light in a direction that is generally lateral with respect to the longitudinal axis of the optical fiber. In particular, this type of delivery device has found wide acceptance in minimally invasive medical applications, including but not limited to the treatment of benign prostatic hyperplasia.
One method of laterally directing light in a fiber optic is to use a prism or a metal or dielectric mirror tilted with respect to the fiber optic longitudinal axis. There are several geometric and thermal limitations to this method. The mirror angle must be sufficiently obtuse, and its distance from the fiber optic sufficiently large enough to minimize the reflected light which passes through the side of the optic. Additionally, the mirror must be large with respect to the fiber diameter and be encased in a protective tube with a window which results in additional fresnel losses. This reflected light can cause undesirable heating of adjacent tissue and can make the aim beam difficult to visualize.
Metal mirrors have low reflectance, less than 95%, and this results in heat generation which limits their use to low power applications. Although high reflectance dielectric mirrors do not generate heat, they suffer from the same inherent geometric problems.
Another method of angularly directing light is to reflect the light from the beveled surface of a fiber tip. The beveled fiber surface totally internally reflects the light when it is in contact with a medium of sufficiently low refractive index. This is the most common technique presently used and is described in U.S. Pat. Nos. 4,740,047 and 5,257,991 Alternatively, the beveled surface can be coated with a reflective metal or dielectric stack. The problem with beveled fibers is that the light reflecting off the beveled face strikes the highly curved surface of the fiber. Rays that strike the periphery of the fiber at glancing incidence are reflected back, leading to low transmission and unacceptable levels of back scattered light. Another problem is that the light passing through the side of the fiber is subject to a very short focal length cylindrical lens, which causes the light to sharply focus and diverge in one dimension.
Although refraction can be used to divert the light, it has certain limitations. For example, it is limited to diverting the light at small angles. Additionally, there are significant reflection losses unless an anti-reflective coating is applied to the beveled surface.
One of the simplest ways of laterally directing light is to bend the optical fiber. However, when the fiber is sharply bent, light escapes along the outer portion of the bend. To prevent these bending losses, the radius of curvature must be so large, greater than 20 times the fiber diameter, that they cannot be used for endoscopic procedures where space is at a premium.
There is a need for an optical delivery device that laterally directs light in a fiber optic to a desired site without causing undesirable heating of adjacent tissue, and without making an aim beam difficult to visualize. Such optical delivery devices are particularly useful in high power applications and produce a minimum of scattered light.